This invention relates to control apparatus for an electric car. More particularly, it relates to an electric car which is field-controlled by a chopper in such a way that, at powering drive of the electric car, the field coil of a d.c. motor is changed-over from a series winding connection to a shunt winding connection, while at braking drive, the field coil is changed-over from the shunt winding connection to the series winding connection.
As seen, for example, in German literature `ELEKTRISCHE BAHNEN,` Heft 12 (Jarhgang 36 1965), page 289, Bild 19-Bild 21, the control apparatus for an electric car of this type can carry out substantially continuously from full field control to weak field control at powering drive and from weak field control to full field control at regenerative braking drive by one set of choppers without changing-over the main circuit of the motor. Another merit is that the change-over from the powering drive to the regenerative braking is very simple.
The control circuit for the electric car at the powering drive is such that, below a rated speed, the d. c. motor is made a series motor by connecting the field coil and armature thereof in series, the chopper being connected in series with the d. c. motor so as to execute full field control by the on-off control of the chopper. When the duty cycle of the chopper has become the maximum value and the electric car has reached substantially the rated speed, the d. c. motor is made a shunt motor in order to render the speed of the electric car still higher. In this case, the field coil is connected in series with the chopper, and the duty cycle of the chopper is controlled so that the field current may become small as compared with the armature current. Thus, weak field control is executed.
On the other hand, at regenerative braking, conversely to the case of the powering drive, the d. c. motor is changed-over from the shunt winding connection to the series winding connection, and the weak field control followed by the full field control is carried out by the on-off control of the chopper.
Since, however, the change-over between the series winding and the shunt winding at the powering drive or the regenerative braking drive is performed upon condition that the duty cycle of the chopper has become a maximum value, the transient change of the field current at the change-over is large. For this reason, a sharp fluctuation in the torque occurs, which results in such problem that a comfortable ride is ruined.